1. Field of the Invention
The present invention is generally concerned with cotyloidal prostheses and is more particularly, but not necessarily exclusively, concerned with the situation in which a cotyloidal prosthesis is used to replace a coxofemoral joint.
2. Description of the Prior Art
Prior art coxofemoral joint prostheses have two complementary parts: namely a hemispherical cup or cotyle adapted to be fixed directly or indirectly by means of a fixing member to the cotyloid cavity of the iliac bone of the patient and a ball-shaped spherical head at the end of a stem adapted to be inserted into the MNO of the patient.
Given the excellent coefficient of friction of a ceramic material such as an alumina ceramic and the biocompatibility of such materials with bone tissue, it has previously been proposed to make both the spherical head and the cup from ceramic.
Even though, with reference to the spherical head, machining a ceramic ball is relatively simple, and consequently feasible at a relatively acceptable cost, this is by no means true of the cup.
Given the accuracy required for perfect congruence with the associated spherical head and given that at present this is a relatively massive component made in one piece, the cost of a ceramic cup is particularly high.
The accuracy of machining required for the cup leads to the use of techniques employed in optics, in this instance abrasive machining using progressively finer abrasives.
Finish machining of the spherical head and the cup is achieved by mutual abrasion, requiring precise pairing of these components, with resulting considerable difficulties in terms of storage and use.
Most coxofemoral joint prostheses are for older patients, in whom there is a particular tendency for eventual detachment of the cup when it is made from ceramic, this being attributable, for example, to poor attachment of the ceramic to the bone tissue in this case. For this reason, ceramic cups are at present usually employed only for younger patients whose bone tissue seems better able to accommodate this material, probably because the damping of normal impacts, such as those occasioned on walking, is better in these patients.
In older patients the cup is therefore usually made from a synthetic material at present, to be more precise from high-density polyethylene, whereas the spherical head is preferably made of ceramic.
Apart from the inevitable production of polyethylene debris which is currently suspected of causing bone damage, coxofemoral joint prostheses including a synthetic material cup have the drawback of a shorter service life than those in which the cup is made from ceramic.
Various studies have shown that, regardless of the materials from which they are made, the spherical head moves progressively deeper into the cup, partly because of cold flow of the synthetic material of the latter and partly because the spherical head wears away the synthetic material.
Incidentally, this is the reason why coxofemoral joint prostheses using a ceramic-ceramic rubbing pair, i.e. in which the cup and the spherical head are both made of ceramic, are increasingly attracting interest, given the requirement for a long service life in younger patients.
In these devices there is no cold flow and there is virtually no wear.
Additionally, there is no risk of production of polyethylene debris.
A general object of the present invention is a cotyloidal prosthesis which, by virtue of improvements to the use of a ceramic-ceramic rubbing pair, provides in a very simple manner an advantageous compromise between coxofemoral joint prostheses with a ceramic cup and those with a synthetic material cup, and which additionally has further advantages.
It is based on the observation that, in bipeds, the parts of the bone components concerned, namely the cotyloid cavity and the femoral head, which are actually loaded are in practise usually restricted to a small portion of the joint crescent, substantially equivalent to the roof only of the cotyloid cavity.
Gravity and the locomotor function cause the pressure of the head of the femur on the cotyloid cavity to be greater at its roof.
The cotyloidal prosthesis of the invention is particularly suitable for a coxofemoral joint.